i. Field of the Invention
This invention relates to an electrolysis system including an electrolytic cell of the diaphragm-type particularly suitable for the production of chlorine and caustic. It relates, more specifically, to an electrolysis system including an improved diaphargm-type electrolytic diaphragm-type and apparatus containing multiple such diaphragm-type unit cells and to a method of operating such system. The present invention also relates to an improved such diaphragm-type electrolysis apparatus and improved electrolysis process using such improved diaphragm-type cells.
II. Description of the Prior Art
The benefits of the use of metal electrodes in the manufacture of chlorine-alkali, chlorate, perchlorates, etc. have been indicated in many publications: Canadian Pat. No. 771,140 issued Nov. 7, 1967 to S. I. Burghardt relates to the advantages of metal electrodes; Canadian Pat. No. 631,022 issued Nov. 14, 1961 to R. G. Cottam and M. G. Derlez relates to improvements in anodes of that type.
Diaphragm-type electrolytic cells have also advanced in performance with the availability of dimensionally stable anodes, e.g., metal anodes. In the chlor-alkali industry these anodes were proven successful commercially after 1966. However, electrolytic cell design has not changed too significantly even while imploying the new anodes.
It has recently been suggested that perfluorinated ion exchange membranes, especially that known by the Trade Mark of Nafion (Du Pont) be used as the diaphragm for chlorine and caustic production using such diaphragm-type electrolytic cells. It was suggested that if such diaphragm were successful in performance, the electrolytic cell employing this type of membrane would substantially eliminate several of the disadvantages of diaphragm cells, e.g., health hazard to personnel from ythe asbestos fibres heretofore used in the diaphragm. This would also obviate the requirement of salt crystallization and separation of catholyte which is not a necessity for the catholyte from the membrane cell. The membrane cell, however, offers challenges in design for efficient utilization of the properties inherent with membranes and the utilization of modules for ease of assembly and maintenance, as well as for optimum conditions.
For example, in chlorine/caustic production, the following are the reactions:
I. Anode reaction: EQU 2C1.sup.- .fwdarw. Cl.sub.2 + 2e.sup.-
Brine is fed into the anode compartment of the cell and the spent brine plus chlorine is released from the compartment and cell.
ii. Cathode reaction: EQU 2H.sub.2 O + 2e.sup.- .fwdarw. 2OH.sup.- + H.sub.2
water is fed into the cathode compartment of the cell and caustic plus hydrogen is released from the compartment and cell.
iii. Membrane:
The membrane provides an ionically conductive impermeable barrier substantially completely to prevent mixing of the gaseous products and electrolytes respectively. The membrane allows sodium ions to pass into the cathode compartment, but largely excludes both chloride and hydroxyl ions (caustic). The caustic thus formed in the cathode compartment is essentially salt free.
The diaphragm-type cells heretofore provided were of the single cell system-type which involves many concentrations and units for commerical production. This type is likely to be high in capital cost, difficult to control, high in maintenance cost and subject to production interruptions. If larger units are used, the amperage load would tend to be high, which increases power equipment cost and generally makes it difficult to design such cells for efficient and safe operations.
It has previously been found that the dimensions of the membrane, i.e., the width and length respectively, linearly expand up to 20% in the cell. This generally results in blockage of product flow and often results in cracking of the membrane. To minimize dimensional increase and also to strengthen the membrane, such membranes are generally available with a polytetrafluoroethylene fabric, e.g. that known by the Trade Mark of Teflon, on one side. The linear increase is less in this case, but still is significant (approximately 3%). The cell voltage tends to increase and in some cases the fabric results in a sealing problem.
It is known that the membrane must effectively divide the anode and cathode compartments without hydraulic leaks of electrolyte from one compartment to the compartment on the opposite side. Even a pinhole leak would, in most cases, be sufficient drastically to reduce the efficiency and would tend to jeopardize successful operation. Thus, a liquid-tight seal is of utmost importance.
It is also known that the membrane will fail with time as will other parts of the electrolyte system. The electrolytic system should, therefore, be of the type that may be easily disassembled and the parts thereof readily replaced.
The electrolyte flow rate should preferably be many times higher than material balance requirement in order to achieve highest efficiency. Minimum flow control is also desirable.
It is also desirable that the electrodes be spaced as closely as possible in order to minimize cell voltage, but they should not be spaced so closely that they block the flow of electrolyte and gaseous products.
It is also known that the current efficiency is drastically reduced at higher strength catholyte because of back migration of hydroxyl into the anode compartment.